Human Physiology
Why This Chapter Matters
Human physiology spans multiple NEET chapters — digestion, breathing, circulation, excretion, locomotion, and neural control. Together these carry 15-20 marks in NEET. This chapter synthesises all systems of the human body.
Core Concepts
1. Digestion and Absorption
Digestive organs and their enzymes:
| Organ | Secretion | Enzyme | Substrate → Product |
|---|
|---|---|---|---|
| Salivary glands | Saliva | Salivary amylase (ptyalin) | Starch → Maltose |
|---|---|---|---|
| Stomach | Gastric juice | Pepsin (inactive pepsinogen → activated by HCl) | Proteins → Peptides |
| Stomach | Gastric juice | Rennin (in infants) | Casein (milk protein) → Paracasein |
| Pancreas | Pancreatic juice | Trypsin, Chymotrypsin | Proteins → Peptides |
| Pancreas | Pancreatic juice | Pancreatic amylase | Starch → Maltose |
| Pancreas | Pancreatic juice | Lipase | Fats → Fatty acids + Glycerol |
| Liver | Bile | (no enzyme) | Emulsification of fats |
| Small intestine | Intestinal juice | Maltase, Lactase, Sucrase | Disaccharides → Monosaccharides |
| Small intestine | Intestinal juice | Peptidases | Peptides → Amino acids |
Absorption: Mainly in small intestine through villi and microvilli (brush border).
Fatty acids and glycerol → absorbed via lacteals (lymph capillaries) → into blood via thoracic duct.
Glucose, amino acids → absorbed directly into blood (portal vein → liver).
2. Breathing and Gas Exchange
Mechanism of breathing:
Inspiration (active): External intercostal muscles + diaphragm contract → thoracic volume increases → pressure decreases → air in.
Expiration (passive): Muscles relax → thoracic volume decreases → air out.
Lung volumes (average values):
Tidal Volume (TV): 500 mL — air in normal breath
Inspiratory Reserve Volume (IRV): 2500-3000 mL — extra air inspired forcefully
Expiratory Reserve Volume (ERV): 1000-1100 mL — extra air expired forcefully
Residual Volume (RV): 1100-1200 mL — air remaining after forceful expiration
Capacities:
Vital Capacity (VC) = TV + IRV + ERV = 3500-4500 mL
Total Lung Capacity (TLC) = VC + RV = 5000-6000 mL
Functional Residual Capacity (FRC) = ERV + RV
Gas exchange:
O₂: 104 mmHg (alveoli) → 40 mmHg (blood) → O₂ diffuses into blood
CO₂: 45 mmHg (blood) → 40 mmHg (alveoli) → CO₂ diffuses out
O₂ transport: 97% as oxyhaemoglobin, 3% dissolved in plasma
CO₂ transport: 70% as bicarbonate ions (HCO₃⁻), 23% as carbamino-haemoglobin, 7% dissolved
Haemoglobin-O₂ relationship:
Bohr Effect: Increased CO₂ / decreased pH / increased temperature → O₂ releases more readily from Hb → more O₂ to tissues.
3. Blood and Circulation
Blood composition:
Plasma (55%): water + proteins (albumin, globulin, fibrinogen) + dissolved substances
RBCs/Erythrocytes (45%): haemoglobin, no nucleus (mature), biconcave disc
WBCs/Leukocytes: immunity (neutrophil, eosinophil, basophil, monocyte, lymphocyte)
Platelets/Thrombocytes: blood clotting
Heart structure:
4 chambers: Right Atrium, Right Ventricle, Left Atrium, Left Ventricle
Valves: Bicuspid/Mitral (left AV), Tricuspid (right AV), Pulmonary (right semilunar), Aortic (left semilunar)
SAN (Sinoatrial Node) = pacemaker of the heart (generates electrical impulse, 72 beats/min)
AVN (Atrioventricular Node) → Bundle of His → Purkinje fibres → ventricle contraction
Cardiac cycle (0.8 seconds at 75 bpm):
Atrial systole (0.1s) → Ventricular systole (0.3s) → Diastole (0.4s)
Stroke volume = 70 mL | Cardiac output = 5 L/min (70 × 72)
Blood pressure: Systolic / Diastolic. Normal = 120/80 mmHg.
4. Excretory System
Nephron structure:
Glomerulus + Bowman's capsule (filtration) → Proximal Convoluted Tubule (PCT) → Loop of Henle → Distal Convoluted Tubule (DCT) → Collecting duct
Filtration: Blood pressure forces fluid through glomerulus. Glomerular Filtration Rate (GFR) = 125 mL/min = 180 L/day.
Selective reabsorption: glucose, amino acids, water, Na⁺ reabsorbed (mostly in PCT).
Secretion: additional H⁺, K⁺, NH₃ into tubule.
Final urine volume: ~1.5 L/day.
Urine composition: Water (95%), Urea (2%), NaCl, Uric acid, Creatinine.
Glucose in urine = Glycosuria (diabetes mellitus — high blood glucose exceeds renal threshold).
Hormonal control:
ADH (Antidiuretic hormone/Vasopressin): increases water reabsorption in collecting duct → concentrated urine.
Aldosterone: increases Na⁺ and water reabsorption in DCT → more water retained.
Atrial Natriuretic Factor (ANF): increases excretion of Na⁺ → counteracts aldosterone.
5. Neural Coordination
Neuron:
Dendrites → Cell body → Axon → Synapse
Resting potential: -70 mV (inside negative)
Action potential: Na⁺ influx → depolarisation (+40mV) → K⁺ efflux → repolarisation
Types of neurons:
Sensory (afferent): receptor → CNS
Motor (efferent): CNS → effector
Relay/interneuron: within CNS
Synapse:
Electrical synapse: direct cytoplasmic connection (fast)
Chemical synapse: neurotransmitter (acetylcholine, noradrenaline, dopamine, serotonin)
CNS: Brain + Spinal cord
PNS: Somatic (voluntary) + Autonomic (sympathetic + parasympathetic)
Sympathetic: fight or flight (noradrenaline)
Parasympathetic: rest and digest (acetylcholine)
PYQs (NEET)
NEET 2023: GFR (Glomerular Filtration Rate) in a healthy adult is approximately:
125 mL/min (or 180 L per day)
NEET 2022: Tidal volume of a normal healthy adult at rest:
500 mL (about 6 L/min × 80 breaths = 0.5 L per breath)
NEET 2021: Which of the following transports maximum CO₂ in blood?
Bicarbonate ions (HCO₃⁻) — approximately 70% of CO₂ transport.
NEET 2020: Sinoatrial node (SAN) is located in:
Wall of the right atrium. It is the primary pacemaker of the heart.
MCQ Practice
Q1. Bile is produced by:
(A) Pancreas (B) Stomach (C) Liver ✓ (D) Gall bladder
[Liver produces bile; gall bladder STORES it]
Q2. Bohr's effect refers to:
(A) Effect of temperature on enzyme activity
(B) Effect of CO₂/pH on Hb-O₂ dissociation ✓
(C) Effect of pressure on gas exchange
(D) Effect of light on photosynthesis
Q3. Erythropoiesis (RBC formation) occurs in:
(A) Liver (B) Spleen (C) Red bone marrow ✓ (D) Lymph nodes
Q4 (Hard). In a normal healthy person, what percentage of filtered glucose is reabsorbed?
100% — all filtered glucose is reabsorbed in PCT via active transport. Glucose in urine = abnormal.